1. Field of the Invention
This invention relates to an improved fixture for holding substrates, especially during precision planarization of the substrate surface, and more particularly to an improved holding fixture for holding flexible substrates.
2. Description of the Prior Art
Precision planarization of thin, flexible substrates, such as glass ceramic substrates and semiconductor wafers commonly used in the semiconductor industry, is extremely difficult. Such substrates readily flex when loaded in a holding fixture and relax when unloaded after the planarization process, causing a out-of-flat condition in the planarized surface. The problem is compounded by the fact that the substrate surfaces may not be parallel, or flat, and that there may be imperfections in either or both surfaces. Similar problems occur during detailed machining to a precisely controlled depth, photo lithography exposure and similar applications.
There have been a number of proposals in the prior art with respect to various designs for vacuum chucks. But none of these proposals is altogether satisfactory for precisely holding flexible substrates during a precision planarization of one of its surfaces. Following are some of the proposals:
U.S. Pat. No. 4,088,312 to Froeck et al., discloses a variable contour securing system. The retaining mechanism includes a spaced array of adjustable spindles mounted on a housing. Each spindle has a base member support cup at one end thereof. A vacuum source is applied t the cups for seating the member adjacent to the cups.
U.S. Pat. No. 4,684,113 to Douglas et al., discloses a universal holding fixture for holding a work piece having a contoured holding surface. The fixture includes a plurality of work piece engaging rods which are individually adjustable in height to thereby reproduce the contour of the contoured holding surface.
U.S. Pat. No. 4,656,791 to Herrington et al., discloses a support apparatus for supporting a workpiece in a high velocity fluid jet cutting operation. The support apparatus comprises a pair of independent support members that cooperate to provide uniform support to the entire area of the sheet during cutting.
Russian Patent No. SU-761-411, discloses a vacuum operated lifting grab device which has a tube locator with split conical bush, threaded sections with nuts and vacuum collector chambers connected to rigid rods.